1. Field of the Invention
The present invention relates to a balanced amplifier and a filter using the same and, more particularly, to a balanced amplifier whose maximum value of an output signal amplitude is larger than that of a conventional amplifier even at a low voltage, and a filter using the same.
2. Description of the Related Art
A balanced amplifier has a gain only for the differential-mode components of an input signal and removes common-mode components. This balanced amplifier can remove noise mixed as common-mode components and double the amplitude of a differential signal as compared with the amplitude of a single-ended signal. Owing to such advantages, the balanced amplifier is widely used in analog-digital mixed integrated circuits and circuits that operate at low voltages. A circuit for removing common-mode components has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 2000-148262) which is configured by a combination of a differential pair and a common-mode feedback (to be referred to as CMFB hereinafter) circuit and removes common-mode components from an input voltage.
In such a conventional circuit, since the common-mode component removing property of the differential pair is used, the number of cascaded transistors which are stacked is limited, and the maximum value of an output signal amplitude is too small when this circuit operates at a low voltage.
For example, in many semiconductor integrated circuits, analog circuits are formed on the same chip on which digital circuits are fabricated. In order to increase the integration degree of circuits, it is advantageous that digital and analog circuits operate at the same voltage. With advances in microprocessing, however, the power supply voltage is further decreased. For example, the operating voltage for a digital circuit in a 0.11-μm process integrated circuit that is expected to be commercialized in the near future is about 1.5V. With a reduction in the size of a process integrated circuit, the operating voltage is expected to further decrease. When the power supply voltage further decreases in this manner, the differential pair cannot exhibit sufficient performance, i.e., a sufficient output amplitude cannot be obtained, when it is operated at the same voltage as that for the digital circuit. Therefore, it may become difficult to obtain a sufficient noise removing effect even if a noise removing circuit is formed by using a balanced amplifier.
It is an object of the present invention to provide a balanced amplifier in which the upper limit of output signal amplitudes is high even at a low voltage and a filter which uses the balanced amplifier and can exhibit sufficient performance even at a low voltage.